It’s time to get ready for the next summer season.

Arianna Zanella, DVM
Despite we are still in winter season, we think that now it’s the right time to think how to deal with the warmest months of the year in the best way. Heat stress is one of the major causes of economic loss in dairy farms and as a consequence, it deserves special emphasis. The problems related to high summer temperatures derive from the inadequate capability of cows to dissipate heat, when endogenous heat (from metabolism to production) comes in addition to heat of exogenous origin (environment). Traditionally, the ideal temperature range for the cows is between -5°C and 24°C, but in order to evaluate correctly the environmental conditions, it is important to take into account relative humidity too. For a long time, THI or Temperature Humidity Index is being used as a heat stress marker. The threshold of heat stress has been fixed at THI equal to 65, even if particularly productive cows can display heat stress symptoms at lower levels.
What happens under heat stress conditions
The increase of THI corresponds to an increasing severity of negative effects (Pic. 1). Direct consequences of this condition can be clearly quantified by observing a heat-stressed cow:
  • Increase in body temperature;
  • Increase in respiratory frequency;
  • Increase in heart rate;
  • Reduction in the time dedicated to rest and rumination.
Under this condition, cows will have significantly low productive and reproductive performance and they will be much more sensitive to common barn diseases (mastitis and hoof diseases). Concerning milk production, worsenings both from a quantitative and a qualitative point of view can be found (proteins, fat, somatic cells). A cow under heat stress requires more maintenance needs and at the same time, reduces the ingestion of dry matter. Hahn (1999) established that the ingestion’s decline starts when environmental temperature reaches 25°C. In a study written in 2003 (West JW et al.) a 0,5kg drop in the ingestion of dry matter has been calculated for each THI point after 72. Consequently, milk loss can reach even 15% of daily production. In Pic.2 the results of an Italian study (Bernabucci et al.,2010) show that a milk loss of about 3 kg/head/day has been observed when THI increases from 62 to 80. Heat stress affects even reproductive performance, reducing both heat manifestation and conception rate. A study that has been conducted for 3 years in 4 companies in north Spain (Garcia Ispierto, 2007) found a decline in the conception rate (CR) from 35% during cold periods to 28% during hotter periods. While regarding pregnancy rate, a 2005 study (Lozano Dominguez RR et al) reported a decrease of PR of about 1% for each THI point over 72.
Possible interventions
This information takes us to think that heat stress is a problem to face both from a managerial point of view and from a nutritional one, but above all from a structural perspective, in order to guarantee cows’ welfare and to minimize economic loss. Nutritional interventions consist in providing a more appetizing and digestible intake, preferably administered in the coolest moments of the day. From a managerial point of view, it is important to avoid overcrowding and to pay more attention to hygiene in bedding areas. The most effective strategies to handle the problem of heat stress are tools employed to reduce the storage of environmental heat (shadowing) and the installation of ventilation systems which, monitoring THI, allow the air movement both in rest areas and along the feeding line. This kind of forced ventilation is automatically activated at low power with THI=60, up to the maximum power with THI of about 65 to obtain an air speed of about 2-2,5m/s. Forced ventilation allows to increase air speed, which reaches animals favoring thermal exchange with the surrounding.  
Picture 1
Picture 2
Several researches have demonstrated that, by using these strategies, it’s possible to minimize heat stress effects with a consequent reduction of the body temperature, respiratory frequency, heart rate and time of rest and rumination. The benefits perceived by dairy cows are clearly quantifiable, since they are noticeable in a greater ingestion of dry matter and an increase in the production and quality of milk. Several studies have also quantified the effects of heat stress reduction on dry cows and pre-partum heifers:
  • The reduction of heat stress on these groups of animals allowed to have new born calves that weigh 2,6kg more than usual and 3,5kg of milk in addition to the normal production during the first 150 days (Wolfenson et al., 1988);
  • Nardone et al., in 2007 have found colostrum containing immunoglobulins 20% lower in animals exposed to a THI higher than 76 during the 3 weeks before the delivery;
  • A study conducted in 2013 (Monteiro et al.) monitored, until their first lactation, two different groups of calves born respectively from heat-stressed cows and not heat-stressed ones. Animals, who were born under heat stress conditions, reached their first lactation with an average production of 27kg/day in 65% of the cases, against 85% of calves born under “ventilated” cows, with an average production of 32kg/day.
Further advantages deriving from the installation of a ventilation system can be observed in the quality of beddings and air:
  • Drying beddings positively affects the amount of hours during which cows rest (Fregonesi et al.,2007);
  • A lower humidity reduces manure fermentation, which is the main cause of dangerous gases for humans and animals.
For many years the research provided us with several scientific evidences about how much heat stress can be a problem for cattle of dairy cows and consequently how much it can negatively affect their productive and reproductive performance. By implementing proper ventilation systems, it is possible to optimize animals’ welfare even during the hottest periods, therefore minimizing losses and obtaining a certain economic response.